1. Field of the Invention
The present invention relates to a beam control device for controlling a laser beam of a semiconductor laser being provided in an optical memory system in which data can be recorded, playbacked and/or erased by applying the laser beam to a magneto-optical disc.
2. Description of the Prior Art
As is well known to those skilled in the art, the magneto-optical disc has a structure such that an amorphous film of rare metal-ferroalloy is deposited on a substrate such as a glass substrate, by spattering and is covered with a magnetic film having an axis of easy magnetization perpendicular to the surface of the magnetic film.
The optical memory system has a recording head for recording data onto the disc, playback or erasing recorded data in which a semiconductor laser is provided for applying the laser beam to the disc.
In this optical memory system, the recording of data is made according to a method as follows. A laser beam focused to a spot of a diameter of about 1 .mu.m is applied to the magnetic film of the disc to raise the temperature of the portion of the disk to which the laser beam is applied, in order to reduce a coercive force at that portion. At the same time, the direction of magnetization is inverted by applying an auxilliary magnetic field externally to the portion having the temperature raised. The erasing method is essentially same to the recording method just mentioned.
The playback method for data recorded is as follows. A linearly polarized light of the laser beam having an intensity weaker than that of the recording light is applied to the magnetic film of the disc on which data has been recorded, the light having been reflected therefrom has a proper inclination of polarization due to the magneto-optical effect of the magnetic film (Kerr effect). The inclination of polarization is converted to the intensity of light by an analyzer. Play-back signals then can be obtained as outputs from a photo-detector which detects the converted intensity of light.
As is apparent from the description mentioned above, the semiconductor laser should be driven at both a high and low level for recording and playbacking, respectively.
Meanwhile, the semiconductor laser has such a temperature dependence that the intensity of laser beam is varied according to the ambient temperature due to which the threshold current thereof is varied. However, if the intensity of laser beam is varied during the recording at a high level, informations are written into the disc wrongly. This lowers the credibility of the optical memory system. This is also similar to the case of play-back of recorded data. Namely, when the intensity of laser beam is varied during the play-back, S/N ratio of play-back signal to noises is lowered to give wrong information.
In order to solve these problems mentioned above, there has been proposed a laser beam controller for the optical memory system as shown in FIG. 5.
According to this laser beam controller, there are provided first and second current sources (b) and (c) for supplying two different driving currents to a semiconductor laser (a) respectively. The first current source (b) is provided for supplying a low power driving current I.sub.1 during the play-back of recorded data. Meanwhile, during the recording of data, the second current source (c) supplies a high power driving current I.sub.2 to the semiconductor laser together with the first current source (b) in order to obtain a laser beam of a high intensity.
When the laser is driven only by the first current source (b), the intensity of laser beam emitted is detected by a photodetector (d) and an output signal is inputted, via a pre-amplifier (e), into a sample-hold circuit (f). This sample-hold circuit (f) is controlled by a sample-hold signal S.sub.1 in a manner such that, when the sample-hold signal S.sub.1 has a high level, data being entered is held and not held when it has a low level. The data signal outputted from the sample hold circuit (f) is compared with a reference voltage given by a standard voltage source (g) at a differential amplifier (h).
The output of differential amplifier (h) is inputted into a low-pass filter (i) and low frequency components are inputted into a power amplifier (j). The power amplifier (j) controls the low current I.sub.1 of the low power current source (b).
If the sample-hold signal S.sub.1 is set at the low level, according to the power control system mentioned above, the beam intensity of the semiconductor laser (a) is kept constant irrespective to the temperature. This control system is referred to APC (Auto-Power Control) below.
During the high power driving mode (recording mode), the sample hold signal S.sub.1 is switched to a high level. Due to this switch, the sample hold circuit (f) holds the data signal, and therefore, APC is frozen.
Further, when the high power driving mode is chosen, a data-record signal S.sub.D of a high level is applied to an AND gate (k) together with the high level sample-hold signal S.sub.1 which is provided for controlling a switching circuit (1). This switching circuit (1) is turned on when the output of AND gate (k) becomes a high level, and therefore, the current I.sub.2 supplied by the high power current source (c) is added to the current I.sub.1 in order to drive the semiconductor laser (a) at the high power level. The reason for freezing the APC is to avoid a possible drop cf the beam intensity during the recording and/or erasing mode.
However, the APC apparently operates even in the high power driving mode. This is on the premises that only the threshold value is varied according to the ambient temperature when considering the driving current to the beam intensity curvature characteristic cf the semiconductor laser. The gradient of the curvature above the threshold value is not varied according to the ambient temperature. Namely, if the low power driving current I.sub.1 is controlled so as to have a higher value than the threshold value, APC can be realized even in the high power driving mode by superposing a constant current on the low power driving current. But these premises are not correct since the gradient of the curve above the threshold is varied according to the ambient temperature and use-time.
The sample-hold circuit (f) employed in the conventional APC shown in FIG. 6 is comprised of a low-pass filter (m) into which the output from a differential pre-amplifier (h) is inputted, memory means (n) which can store the output V.sub.0 of the low-pass filter (m) and selecting means (o) for switching either the application of the output of the low pass filter (m) or the memory circuit (n) to a low power driving current source (b). When the play-back mode is indicated, the switching means (o) is switched so as to connect the low-pass filter (m) to the low power driving current source (b) directly. The differential amplifier (h) outputs a signal V.sub.1 proportional to the difference between the output signal Va from the photo-detector (d) and a predetermined reference voltage Vb. Therefore, APC is obtained as mentioned above.
When the recording mode (high power driving mode) selected, the switching circuit (o) is switched by the data recording signal S.sub.D so as to connect the memory circuit (n) to the low power driving current source (b). Accordingly, the sample hold circuit (f) outputs a voltage signal V.sub.m having been stored in the memory circuit (n) due to the sample-hold signal S.sub.1. As is clearly understood, the sample-hold circuit (f) has a first mode in which the output V.sub.0 is outputted via the low pass filter (m) and a second mode during which the output V.sub.m stored in the memory circuit (n) is outputted.
However, this conventional sample-hold circuit (f) has an essential disadvantage in that it is difficult to obtain a quick response upon switching from the recording mode to the play-back mode or vice versa since the low pass filter (m) has a slow transition response.
As shown in FIG. 7, the low pass filter (m) has a relatively slow transition response. When the mode is switched from the recording mode to the play-back mode, the transition of the output voltage Vo of the low pass filter (m) is delayed. Due to this delay in the transition response, the low power driving current I.sub.1 supplied by the first current source (b) is also delayed, and therefore, the stabilization of the low power driving current I.sub.1 is delayed.